Ink compositions which are erasable with a rubber eraser after writing (hereinafter referred to as "erasable inks") are known. For example, Japanese Unexamined Patent Publication No. Sho 59-223769 discloses "an ink composition which comprises a pigment selected from carbon black and aniline black and homogeneously dispersed in an aqueous medium having dissolved therein polyethylene oxide in the presence of a surfactant containing a polyoxyethylene group." The disclosed erasable ink, however, is poor in stability, unsatisfactory in erasability itself and difficult to erase after the lapse of long term.
Japanese Patent Application No. Hei 1-217088 filed by the present applicant discloses "an ink composition which is erasable with a rubber eraser, comprising 1 to 50% of a pigment, 3 to 50% of a resin having a film-forming temperature of not lower than 40.degree. C., 0.5 to 50% of an organic solvent and 7 to 60% of water, based on the total weight of the ink composition". The above ink (which may be hereinafter referred to as "ink of prior invention") is markedly superior to conventional erasable inks in, e.g. flowability, erasability and stability. Yet the ink of prior invention tends to have a slightly low stability to dispersion over a long term because a pigment is used as a coloring agent. More specifically, if a ball-point pen filled with the above ink has been left to stand upright, the ink fed to the substrate for writing may become blurred due to the sedimentation of pigment.